The technical problem relates to providing aircraft, particularly drones, with a radar function for the non-cooperative detection of aerial obstacles. This function is essential for allowing autopiloted aircraft to fly in a non-segregated aerial region. It forms part of what is called the “sense-and-avoid” function.
Such a radar must have a very wide field of observation (typically±110° in azimuth and ±15° in elevation) and must be capable of scanning space in a very short time, of the order of magnitude of one second, owing to the time needed to undertake an avoidance manoeuvre if there is a risk of collision. These characteristics correspond approximately to the capability of a human pilot to observe the environment, as required for a “see-and-avoid” function.
For such an application, it is advantageous to use a transmit antenna or several transmit antennas with a wide field and a plurality of receive antennas with a wide field and to form, on receive, many beams simultaneously in the reconnoitred region: this is a technique known as DBF (digital beam forming).
This solution is conventionally implemented using antenna arrays, the radiation patterns of which must have sufficient directivity to locate targets with high precision.
In the case of a sense-and-avoid application, the main problems that the antenna system, and more generally the radar system, has to solve are notably, in the case of the radar system:
a minimum pre-collision warning time of around 20 seconds for targets having a relative radial velocity RRV of between about 0 and 400 m/s, the RRV being positive for a closing target;
a long range in the case of a fast-moving target, that is to say the Doppler frequency of which is located outside that of the ground clutter; and
a shorter range in the case of a slow-moving target, but with good visibility capability with ground clutter present.
Hereafter, by definition, the relative radial velocity or RRV is positive when it corresponds to the target coming closer.
For the antenna system, the main problems are notably:
the T/R (Transmit/Receive) directivity must be better than 10° in the two planes for location precision;
the antenna radiation patterns must have side lobes that are as small as possible so as to reject ground clutter, in particular during low-altitude flight phases and when the target has a low velocity;
moreover, the area of the antenna must be large enough to ensure range efficiency with a reasonable transmit power, typically 20 watts RMS;
in addition to these technical constraints, the radar must be able to be fitted onto various types of aircraft, and the constraints in terms of electronics volume and surface available for the antenna are extremely tight; and
finally, the overall cost of the antenna electronics must be minimized.
No radar device meeting all these requirements exists since the current systems:
either do not meet the requirement in terms of measurement replenishment rate and angular extent. In general, radars are provided for other applications, notably for missile homing heads or small navigation radars, it being attempted to adapt these to the sense-and-avoid function problem;
or meet the requirement in terms of angular coverage using electronic scanning; it should be noted that such a system is expensive and, if narrow beams are used, needed for the measurement precision, it must have only a very brief target observation time to meet the constraint relating to measurement replenishment rate. Under these conditions, Doppler processing is not really conceivable, since the Doppler resolution is too coarse, or is even impossible;
or meet the requirement in terms of angular coverage and precision, using conventional DBF techniques with a two-dimensional receive antenna fully equipped with receive modules; it should be noted that such a system is expensive due to the number of modules necessary to achieve the required precision.
Systems operating on the MIMO (multiple inputs-multiple outputs) principle use a multiple-access antenna array as inputs and outputs. An overall antenna system in the form of a cross has notably been described in the article presented at the RADAR 2008 conference in Adelaide (Australia) by Professor John Roulston in September 2008: “The Post-War Development of Fighter Radar in Europe—A British Perspective”. As such, this system uses a plurality of codes and their implementation can prove to be very expensive without the particular features of the invention. Another system intended to address the S&A problem, using the MIMO principle, has also been described, during the 5th EMRS DTC Technical Conference in Edinburgh, July 2008: “Design Studies for an Airborne Collision Avoidance Radar” by Dennis Longstaff, Mostafa Abu Shaaban and Stefan Lehmann, Filtronic Pty Ltd, Brisbane, Australia. However, such a system has the following drawbacks:
its range is too short with respect to the pre-warning time needed to perform an avoidance manoeuvre; and
it specifically deals with avoiding targets in a collision path, but does not solve the problem of horizontal and vertical traffic separation.